1. Field of the Invention
The present invention relates to a booster circuit which generates a high voltage, and which is used for a driver IC, a non-volatile semiconductor memory, a semiconductor integrated circuit and the like.
2. Description of the Related Art
Conventional booster circuits may be categorized into two types: one using a coil, and the other using a capacitor element. The booster circuit using a capacitor element is generally termed a “charge pump”. In a conventional booster circuit, a large capacitor for boosting an output voltage is connected to a node VOUT, and an NMOS transistor, termed a transfer gate transistor, is inserted between a node VIN and the node VOUT, where VIN denotes an input voltage and VOUT denotes an output voltage. As a result, the node VIN and the node VOUT are separated from each other by the transfer gate transistor.
One electrode of a capacitor for boosting a gate voltage is connected to a gate electrode of the transfer gate transistor. The other electrode of the capacitor for boosting a gate voltage is connected to a clock terminal. Thereby, an appropriate clock signal is inputted to the electrode.
A switching transistor is connected between the node VIN and the gate electrode of the transfer gate transistor. Consequently, while the switching transistor is turned on, the potential of the gate electrode of the transfer gate transistor and the potential of the node VIN are equal to each other.
To reduce a leakage current in the booster circuit, a high-threshold NMOS transistor is used as the transfer gate transistor. In addition, a substrate bias effect caused by a boosted voltage further raises the threshold of the transfer gate transistor.
A high voltage needs to be applied to the gate electrode of the transfer gate transistor in order to operate such a high-threshold transfer gate transistor. For raising a voltage applied to a node (hereinafter referred to as “node A”) to which the gate electrode of the transfer gate transistor is connected, it is necessary to raise a voltage applied to a node (hereinafter referred to as “node B”) to which the gate electrode of the switching transistor is connected.
The voltage VOUT outputted from the booster circuit depends on a power supply voltage VDD, the temperature, and dispersion among the thresholds of the respective transistors used in the booster circuit. Nevertheless, the booster circuit needs to output a voltage at a required level under any conditions. Specifically, a high voltage needs to be applied to each of the nodes A and B in order to turn on the transfer gate transistor even under conditions in which the power supply voltage VDD is low, the temperature is low, and the thresholds of the respective transistors used in the circuit are high.
However, in such a conventional circuit as described above, an excessively high voltage may possibly be applied to the node B in the case where the power supply voltage VDD is high, where the temperature is high, and where the thresholds of the respective transistors used in the circuit are low. Depending on the breakdown voltage limits of junctions respectively in the transfer gate transistor and the switching transistor, such an excessively high voltage might cause a problem that any of the transistors malfunctions and/or a problem that junction leakage occurs therein.